Analysis of a ³²P material balance method for measuring periphyton production in flowing water

Graduation date: 1975The ³²P material balance method for measuring periphyton production and grazing rates on periphyton in streams developed by Elwood and Nelson (1972) was evaluated by laboratory and field experiments. Three basic assumptions of the method were examined: 1) sloughing of periphyton from substrates is negligible, 2) sorption of ³²P onto substrates is a biological process, and 3) recycling of ³²P into the periphyton after ingestion by grazers does not occur. Sloughing of periphyton from stream substrates was assumed negligible except in the case of freshets and dense algal mats. Sorption of ³²P onto stream substrates was mainly a biological process except on very fine sediments. Sorption of ³²P onto leaf material was a function of microbial activity, which was a function of residence time of the leaves in the stream. Recycling of ³²P into the periphyton after ingestion by grazers was a potential problem for estimating grazing rates in slow current. ³²P material balance method was verified against the change-in-standing crop method in the laboratory. The estimates of 576 mg/m²/day and 522 mg/m²/day respectively were not significantly different at the 95% confidence level. The application of the ³²P material balance method in the field was not successful. This difficulty was most likely due to the low retention time in the high gradient Cascade stream in which it was applied. The high cost of the method, extensive sampling time, and difficulties in obtaining authorization in streams with public access warranted the development of a suitable modification. The proposed modification involves the incubation of individual substrates within a net enclosure in the stream